What Those White Lines Behind Airplanes in the Sky Really Are

If you’ve ever looked up at the sky and watched a plane fly by, you’ve probably noticed a long white cloud streaming behind it and wondered what that line is. I spoke with Amanda Martin, the acting warning coordination meteorologist and senior aviation meteorologist at the Aviation Weather Center, to learn precisely what contrails are and how they impact the environment. “Contrails are condensation trails left behind a plane, composed of tiny droplets of water and by-products of jet fuel combustion,” explains Martin. “Contrails are a type of human-made cloud.”

How Contrails Are Formed

Sometimes, you look up at the sky near an airport and see dozens of contrails, while other times, you don’t see any at all. That’s because it takes specific atmospheric conditions to create contrails. “Contrails are formed when the hot air from the plane's exhaust mixes with the cooler temperature and lower pressure environment at flight level,” says Martin. “This mixing occurs directly behind the plane and is visible as a contrail.” 

The air needs to be both cool and humid for contrails to form. If humidity is low, the contrail will evaporate quickly. If humidity is high, contrails will persist and grow. The water vapor from aircraft exhaust turns to ice crystals and combines with water already in the air to create these clouds. They can grow several miles long and 600 to 1,300 feet tall when persistent. 

Related: Why Airplanes Are Almost Always Painted White

Types of Contrails

There are three main types of contrails: short-lived, persistent non-spreading, and persistent spreading. “The only difference to contrails is in regards to their duration, and this is dependent on the large-scale weather pattern,” elaborates Martin. “When contrails are persistent spreading, they will start to resemble cirrus clouds.” 

Short-lived contrails aren’t cause for much concern, as they dissipate quickly, but persistent contrails may be an environmental concern, as they can trap the heat that should be released and ultimately contribute to climate change. 

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Environmental Impact of Contrails

According to the Environmental Protection Agency (EPA) and Federal Aviation Administration (FAA), contrails don’t directly threaten the public, but they may contribute to human-induced climate change. According to a study released in January 2021, contrails make up 57 percent of global aviation’s contribution to climate change. However, most of these contrails are formed by a small percentage of flights. As such, it would be relatively simple to mitigate the impact of contrails by altering the routes of flights that are most likely to produce contrails. They require specific atmospheric conditions that can be avoided when planned for. 

According to another study released just a few months later in May 2021, it would cost less than $1 billion per year to prevent most of the damaging impacts of contrails, while the benefit would be worth more than 1,000 times that cost. According to a study released in March 2024, “navigational contrail avoidance will require minimal additional cost (0.08 percent) and fuel (0.11 percent) investments to achieve notable reductions in contrail climate forcing (−73.0 percent).” 

Of course, the airline industry is always innovating, and companies are already looking for ways to mitigate the climate impacts of contrails. Airlines like Air France are addressing these environmental concerns by conducting thorough research and tests to identify when and where contrails are most likely to form and developing ways to avoid these areas. Breakthrough Energy partnered with Google Research and American Airlines to develop software that plans flight routes to avoid areas where persistent contrails are likely to form.